The present invention relates to a surface reflector having a surface reflecting multilayer film for use with optical instruments such as a camera, a telescope and a microscope.
Aluminum is commonly used as a reflective material for surface reflectors used in optical instruments. However, single-layered films of aluminum have the disadvantage that they are low in mechanical strength, film adhesion to the substrate, moisture resistance and the like. To solve this problem, a protective layer is formed on the single-layered aluminum film and the resulting multilayer arrangement has improved mechanical strength, film adhesion to the substrate and moisture resistance.
For example, FIG. 1 shows a conventional two-layer arrangement which comprises a substrate 1c that is overlaid with a reflecting aluminum layer 3c which in turn is overlaid with a protective layer 4c made from aluminum oxide.
Another two-layer arrangement is shown in FIG. 2 and it comprises a substrate 1d that is overlaid with a reflecting aluminum layer 3d which in turn is overlaid with a protective layer 4d made form silicon dioxide.
Further, a conventional three-layer arrangement is shown in FIG. 3 and it comprises a substrate 1e that is overlaid with a chromium undercoat 2e which in turn is overlaid successively with a reflecting aluminum layer 3e and a protective layer 4e made from aluminum oxide.
Each of these arrangements will prove effective when the substrate of the surface reflector is made of glass.
Also, silver having high reflectance over the visible to near infrared range is commonly used as a reflective material for high-reflectance surface reflectors used in optical instruments. However, single-layered films of silver have the disadvantage that they are low in film adhesion to substrate, moisture resistance, sulfur resistance and the like. To solve this problem, an undercoat is formed between a substrate and a reflecting silver layer, and a protective layer is formed on the reflecting silver layer. The resulting multilayer arrangement has improved film adhesion to substrate, moisture resistance, sulfur resistance and the like.
For example, FIG. 4 shows a conventional four-layer arrangement which comprises a substrate 1f that is overlaid with an aluminum oxide undercoat 2f which in turn is overlaid with a reflecting silver layer 3f. Over the reflecting layer 3f, an aluminum oxide layer 4f and a silicon dioxide layer 5f are successively formed as protective layers [see "Reflectance and durability of Ag mirrors coated with thin layers of Al.sub.2 O.sub.3 plus reactively deposited silicon oxide" in Appl. Opt., 14 (1975), 2639].
FIG. 5 shows a conventional six-layer arrangement which comprises a substrate 1g that is overlaid with a copper undercoat 2g which in turn is overlaid with a reflecting silver layer 3g. Over the reflecting layer 3g, an aluminum oxide layer 4g, a tantalum oxide layer 5g, a silicon dioxide layer 6g and a tantalum oxide layer 7g are successively formed as protective layers [see "Progress in the development of a durable silver-based high-reflectance coating for astronomical telescopes" in Appl. Opt., 24 (1985), 1164].
Each of these arrangements will prove effective when the substrate of the surface reflector is made of glass.
With the recent advances in the technology of molding plastics including polycarbonates, polyesters, acrylic resins and the like, and with the advantage of plastics that they can be molded into complex shapes more easily than glass, the use of plastics in optical parts has increased.
For example, polygonal mirrors used in a laser scanner and other optical devices are desirably made from plastics in order to reduce the device's cost and weight. Also, pentaprisms as camera parts are desirably made from plastics in order to reduce the cost of the camera, and products having a surface reflecting film formed in a hollow pentagonal molded article have begun to be used. However, surface reflectors using a substrate made of plastics such as polycarbonates, polyesters or acrylic resins have the disadvantage that they have lower film adhesion to the substrate, moisture resistance, sulfur resistance and the like than reflectors using a glass substrate if they adopt the multilayer arrangements described above.